But a study from the University of New South Wales' School of Public Health and Community Medicine has identified additional risks.

The results have been published today in the journal Sexually Transmitted Infections, part of the British Medical Journal group.

Study leader Bette Liu says the research team matched the birth records for more than 350,000 women in New South Wales with legally notifiable infections such as gonorrhoea and chlamydia.

About 3,650 women of the women, just more than 1 per cent, had a chlamydia notification.

"They had about a 17 per cent increase in the likelihood that they would have a spontaneous pre-term birth before 37 weeks and they had about a 40 per cent increased probability of having a still birth," Dr Liu said.

The study also found a doubling of premature births for woman with the less prevalent sexually transmitted infection gonorrhoea, compared to those with no record of infections.

The study was not restricted to infections during pregnancy. More than 80 per cent of the women had been diagnosed and ordered treatment before their estimated dates of conception.

"Certainly for chlamydia, women who had a diagnosis during the pregnancy, compared to women who had a diagnosis before the pregnancy, they both had an increased risk of having a spontaneous pre-term birth, and that risk wasn't really different depending on the timing of the infection," Dr Liu said.

She says the heightened risk was still apparent when other factors connected to birth problems, such as social disadvantage, age, smoking, race, diabetes or high blood pressure, were taken into account.

"It does support a number of other studies that have also suggested these infections can increase the chances of poor birth outcomes," she said.

The findings are not proof of cause and effect, but Dr Liu says the study can improve public health messages.

"Women need to be aware that there may be implications for future births," she said.

The Department of Health and ageing says 80 per cent of chlamydia cases are reported in 15 to 29 year olds.

"It certainly underlies that public health message about the importance of safe sex practices for young people to use condoms when they have new partners," Dr Liu said.

"Just to actually prevent infections is a really important message coming out from this research."